^^^ But the mode control is rotary and two clicks from 'normal' to get to STBY
That picture is from FCOM1.
The normal position for the switch is TA/RA.
It NEVER leaves that position except when called for by the Checklist (normally following an engine failure. Then it goes from TA/RA to TA ONLY).
It is completely inconceivable to me that any 777 pilot can accidentally select standby on this type of XPDR.
Switching it to STBY will also create an TCAS advisory message on EICAS.
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WRT Emer Descent and the XPDR:
There is no changing of the XPDR mode in ANY checklist associated with Emer Descent.
The Emer Descent checklist only calls for 7700 set.
This is normally accomplished by the PM (we are trained to do it at the same time as we transmit a MAYDAY message).
The PF will be getting the aircraft in a descent and will not touch the XPDR as he's too busy with flying.
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Disclaimer: I do not fly 777s for MAS